Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells.
A thin-film photovoltaic or optoelectronic device is ordinarily manufactured by depositing material layers onto a substrate. A thin-film photovoltaic device ordinarily comprises a substrate coated by a layer stack comprising a conductive layer stack, at least one absorber layer, optionally at least one buffer layer, and at least one transparent conductive layer stack.
The present invention is concerned with photovoltaic devices comprising an absorber layer generally based on an ABC chalcogenide material, such as an ABC2 chalcopyrite material, wherein A represents elements in group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(In,Ga)Se2 semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indium-selenide or copper-gallium-selenide, in the form of quaternary, pentanary, or multinary materials such as compounds of copper-(indium, gallium)-(selenium, sulfur), copper-(indium, aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium, gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium, sulfur).
The photovoltaic absorber layer of thin-film ABC or ABC2 photovoltaic devices can be manufactured using a variety of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spraying, sintering, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple evaporation sources. Historically derived from alkali material diffusion using soda lime glass substrates, the effect of adding alkali metals to enhance the efficiency of thin-film ABC2 photovoltaic devices has been described in much prior art (Rudmann, D. (2004) Effects of sodium on growth and properties of Cu(In,Ga)Se2 thin films and solar cells, Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved 2012-09-17 from <URL: http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf>).
Much prior art in the field of thin-film ABC2 photovoltaic devices mentions the benefits of adding alkali metals to increase photovoltaic conversion efficiency and, of the group of alkali metals comprising elements Li, Na, K, Rb, Cs. Best results have been reported when diffusing sodium from precursor layers (see for example Contreras et al. (1997) On the Role of Na and Modifications to Cu(In,Ga)Se2 Absorber Materials Using Thin-MF (M=Na, K, Cs) Precursor Layers, NREL/CP-520-22945), or also EP0787354 by Bodegaard et al., or as well US20080023336 by Basol). More recent prior art provides data regarding diffusion of sodium and potassium from an enamelled substrate while also mentioning that potassium is known to dope CIGS in a similar way as sodium and hinders the interdiffusion of CIGS elements during growth of the absorber layer (Wuerz et al. (2011) CIGS thin-film solar cells and modules on enamelled steel substrates, Solar Energy Materials & Solar Cells 100 (2012) 132-137). Most detailed work has usually focused on adding or supplying sodium at various stages of the thin-film device's manufacturing process For reference, the highest photovoltaic conversion efficiency achieved in prior art for a photovoltaic cell on a polyimide substrate, i.e. on a alkali-nondiffusing substrate, with an ABC2 absorber layer where sodium is added via physical vapor deposition of NaF, is about 18.7%, as reported in Chirila et al. (2011) Nature Materials 10, 857-861.
The controlled addition of potassium from sources external to a potassium-nondiffusing substrate has also been described in WO 2014/097112 of the present applicants.